Investment casting techniques innovated in the late 1960s have revolutionized the design, construction and performance of golf club heads up to the present time. Initially only novelty putters and irons were investment cast, and it was only until the early years of the 1980s that investment cast metal woods achieved any degree of commercial success. The initial iron club heads that were investment cast in the very late 1960s and early 1970s innovated the cavity backed club heads made possible by investment casting which enabled the molder and tool designer to form rather severe surface changes in the tooling that were not possible in prior manufacturing techniques for irons which were predominantly at that time forgings. The forging technology was expensive because of the repetition of forging impacts and the necessity for progressive tooling that rendered the forging process considerably more expensive than the investment casting process and that distinction is true today although there have been recent techniques in forging technology to increase the severity of surface contours albe them at considerable expense.
The investment casting process, sometimes known as the lost wax process, permits the casting of complex shapes found beneficial in golf club technology, because the ceramic material of the mold is formed by dipping a wax master impression repeatedly into a ceramic slurry with drying periods in-between and with a silica coating that permits undercutting and abrupt surface changes almost without limitation since the wax is melted from the interior of the ceramic mold after complete hardening.
This process was adopted in the 1980s to manufacture "wooden" club heads and was found particularly successful because the construction of these heads requires interior undercuts and thin walls because of their stainless steel construction. The metal wood club head, in order to conform to commonly acceptable club head weights on the order of 195 to 210 grams when constructed of stainless steel, must have extremely thin wall thicknesses on the order of 0.020 to 0.070 inches on the perimeter walls to a maximum of 0.125 inches on the forward wall which is the ball striking surface. This ball striking surface, even utilizing a high strength stainless steel such as 17-4, without reinforcement, must have a thickness of at least 0.125 inches to maintain its structural integrity for the high club head speed player of today who not uncommonly has speeds in the range of 100 to 150 feet per second at ball impact.
Faced with this dilemma of manufacturing a club head of adequate strength while limiting the weight of the club head in a driving metal wood in the range of 195 to 210 grams, designers have found it difficult to increase the perimeter weighting effect of the club head.
In an iron club, perimeter weighting is an easier task because for a given swing weight, iron club heads can be considerably heavier than metal woods because the iron shafts are shorter. So attempts to increase perimeter weighting over the past decade have been more successful in irons than "wooden" club heads. Since the innovation of investment casting in iron technology in the late 1960s, this technique has been utilized to increase the perimeter weighting of the club head or more particularly a redistribution of the weight of the head itself away from the hitting area to the perimeter around the hitting area, usually by providing a perimeter wall extending rearwardly from the face that results in a rear cavity behind the ball striking area. Such a club head configuration has been found over the last two plus decades to enable the average golfer, as well as the professional, to realize a more forgiving hitting area and by that we mean that somewhat off-center hits from the geometric center of the face of the club results in shots substantially the same as those hits on the center of the club. Today it is not uncommon to find a majority of professional golfers playing in any tournament with investment cast perimeter weighted irons confirming the validity of this perimeter weighting technology.
Metal woods by definition are perimeter weighted because in order to achieve the weight limitation of the club head described above with stainless steel materials, it is necessary to construct the walls of the club head very thin which necessarily produces a shell-type construction where the rearwardly extending wall extends from the perimeter of the forward ball striking wall, and this results in an inherently perimeter weighted club, not by design but by a logical requirement.
In the Raymont, U.S. Pat. No. 3,847,399 issued Nov. 12, 1974, assigned to the assignee of the present invention, a system is disclosed for increasing the perimeter weighting effect of a golf club by a pattern of reinforcing elements in the ball striking area that permits the ball striking area to be lighter than normal, enabling the designer to utilize that weight saved on the forward face by adding it to the perimeter wall and thereby enhancing perimeter weighting.
This technique devised by Mr. Raymont was adopted in the late 1980s by many tool designers of investment cast metal woods to increase the strength of the forward face of the metal woods to maintain the requirement for total overall head weight and to redistribute the weight to the relatively thin investment cast perimeter walls permitting these walls to not only have greater structural integrity and provide easier molding and less rejects, but also to enhance the perimeter weighting of these metal woods. Most major companies in the golf industry manufacturing metal woods in the late 1980s were licensed under the Raymont patent.
In 1991, the Allen, U.S. Pat. No. 5,060,951 issued entitled "Metal Headed Golf Club With Enlarged Face", also assigned to the assignee of the present invention, and it discloses an investment cast metal wood with an enlarged club face depth(height) on the order of at least 1.625 inches. Such a face depth was not formerly believed possible because of the requirement for face structural integrity under the high impact loads at 100 to 150 feet per second, and the weight requirement of the club head of 195 to 210 grams. In this Allen patent, a labyrinth of reinforcing elements similar to Mr. Raymont's was utilized not to re-distribute face weight but instead to enlarge face area while maintaining overall club head weight. An ancillary and important advantage of this development, utilized by many present day designers of "jumbo" metal wood heads, is the fact that an enlarged club face produces a sweet spot enlargement far greater than the enlargement of the club face itself.
There are, however, limitations on the effectiveness of the reinforcing elements on the face wall of investment cast clubs and particularly metal woods. Because investment cast metal woods must have hollow interiors, these interiors must be formed by removable core pieces. To the present day face wall reinforcement has been effected in accordance with the above Raymont and Allen patents by forming integral ribs and bars on the rear surface of the forward ball striking wall. In order to effect this rib pattern, the core pieces that form the rear surface of the ball striking wall, as well as the ribs themselves, must be withdrawn rearwardly in order to clear the ribs. However, the perimeter wall extending rearwardly from the forward wall inhibits the direct rearward removal of these core pieces from the forward wall during the casting operation. Therefore, it has been commonplace to either make these reinforcing elements very shallow on the order of 0.030 to 0.050 inches in rearward depth or to rearwardly taper the ribs almost to a point extending rearwardly from the forward face so that these core pieces can move laterally somewhat as they are removed from the forward wall at the completion of the casting cycle.
These limitations detract from the effectiveness of the reinforcing elements and their capability of achieving a lighter front ball striking wall. As described in the Raymont patent, the effectiveness of the reinforcement of the forward wall is determined by the "I" or "T" beam configuration of the reinforcing elements. The amount of reinforcement is determined in part by the depth and width of the reinforcing walls in a plane transverse of the ball striking wall at its point furthest from the ball striking wall. In an "I" beam configuration, the width of the cross piece away from the forward wall, can be selected as desired but is extremely difficult to mold because of the undercut on the rear web. Such increase in web width and augmentation of the depth of the reinforcement has not to this date been possible prior to the present invention, and hence the full advantages of increased perimeter weighting, superior face reinforcement, and face enlargement have not been thus far fully exploited.
Another problem addressed by the present invention is the achievement of increasing the benefits of perimeter weighting by simply adding weight to the perimeter of the club head itself. This technique of course has found considerable success in low impact club heads such as putters, where overall club head weight is in no way critical, and in fact in many low impact clubs that have found considerable commercial success, the club heads weigh many times that of metal wood heads, sometimes three or four times as heavy.
To this date, however, increased perimeter weighting has not been found easy because of the weight and impact strength requirements in metal woods. An understanding of perimeter weighting must necessarily include a discussion of the parameter radius of gyration. The radius of gyration in a golf club head is defined as the radius from the geometric or ball striking axis of the club along the club face to points of club head mass under consideration. Thus, in effect the radius of gyration is the moment arm or torquing arm for a given mass under consideration about the ball striking point. The total moments acting on the ball during impact is defined as the sum of the individual masses multiplied by their moment arms or "radii of gyration". And this sum of the moments can be increased then by either increasing the length of the individual moment arms or by increasing the mass or force acting at that moment arm or combinations of the two.
Since it is not practical, except for the techniques discussed in the above Raymont and Allen patents, to add weight to the perimeter wall because of the weight limitations of metal woods and particularly the driving woods, one alternative is to increase the moment arm or radius of gyration. This explains the popularity of today's "jumbo" woods although many of such woods do not have enlarged faces because of the requirement for structural integrity in the front face.
In the Allen, U.S. patent application, Ser. No. 882,561, some of these problems are solved with a composite face wall that includes an impact supporting wall that is investment cast with the remainder of the head(without the sole plate which is a separate piece as cast). This impact supporting wall is rigidified by a pattern of integrally cast reinforcing bars that extend forwardly from the forward wall rather than rearwardly as described in the above discussed Raymont and Allen patents. This reinforcing pattern has a depth of approximately 0.150 inches which is significantly greater than reinforcing patterns possible on the rear of the ball striking faces of prior constructions. This increased depth provides far greater supporting wall reinforcement. It is also easily cast because the core piece that forms these deep depth reinforcing elements are removed by a direct forward withdrawal unencumbered by the perimeter wall that inhibits rearward core withdrawal inside the club head. In the exemplary embodiment of this pattern of reinforcing bars, the reinforcing bars are formed into hexagonal unit cells having a major diameter of 0.500 inches, although other geometric patterns are within the scope of the present invention.
This reinforced supporting wall is covered by a very hard plastic ball striking insert that is cast in situ (in place) over the supporting wall. That is, after the head is investment cast, the forward wall is cleaned and vulcanized with a bonding agent and placed in a mold that carries the configuration of the outer surface of the insert and an elastomeric material is either poured or injected under pressure into the mold to form the insert. One material that has been found successful is a Shore D 75 hardness polyurethane, which results in a very hard high frequency ball striking surface. This plastic insert, not only provides a very hard ball striking surface, but more importantly because it is intimately bonded to the forward wall and the reinforcing bars, it provides an effective "I" beam support with the bars for the forward wall as opposed to a "T" beam support found in today's rearwardly reinforced ball striking wall. It can be easily demonstrated by engineering calculation that I beam supports for transverse loads are substantially stronger than T beam supports.
The increase in the radius of gyration is accomplished by extending the heel and toe portions of the club head further from the geometric center of the face wall, beyond present day parameters for high impact club heads. These extensions provide greater effective heel and toe weighting. The heel of the club head is formed by extending the club face significantly beyond the hosel, that is, on the side of the hosel opposite the ball striking area, and extending the top wall and rear wall to accommodate this extended face. These extensions of the heel and toe are accomplished without any significant increase in overall club head weights, by extending the club head top wall downwardly almost to the plane of the sole plate, and flattening the rear wall almost to the plane of the sole plate. This design reduces perimeter wall and sole plate wall weight for a given size head and enables the saved weight to be positioned at the extended heel and toe portions of the club head.
It is a primary object of the present invention to ameliorate problems described above in the manufacture of golf clubs.